Amplifier circuit



May 15, 1934- c. A. TILEsToN ET A1.

AMPLIFIER CIRCUIT Filed Dec. l5, 1928 A` the desired volume.

Patented May 15, 1934 UNITED STATES AMPLIFIER CIRCUIT Clarence A. Tileston, West Orange, N. J., and Rufus H. Caldwell, Astoria, N. Y.

Application December 13, 1928, Serial No. 325,732

1 Claim.

l0 when reproduced, is transferred to a photo-electric cell which in turn gives out a correspondingly variable electric current which, however, is very small and feeble. With other forms of reproduction, such as from a disk record, the uc- .15j tuation on the record groove operates the armature of an electromagnetic or other form of pickup which in turn gives out the feeble electric current. Similarly with the reception of broadcast radio waves, the detector gives out such a small current that it is not suiiicient to cause an audible sound.

It is therefore necessary to amplify this current to such an amount that a suiicient volume of energy is created to reproduce the sound in In talking motion picture reproduction, it is also necessary that the amplication be with the very minimum of distortion and with the very least possibility of failure. Duplicate ampliiiers, however, are extremely expensive and one of them is not in use substantially all of the time.

It is an object of the present invention to produce a sound amplifier which is adapted to amplify electric currents of the magnitude produced by a photo-electric cell, electro-magnetic or electro-static pick-up operated from a sound film or disk to such proportions as will be necessary in a moving picture theatre and one which will always be in operation, kbut which will always have the necessary reserve capacity to meet with possible break-down of individual units such as vacuum tubes.

It is a further object of this invention to provide a sound amplifier to amplify the output of a photo-electric cell or other pick-up devices for use in motion picture theatres which will be infallible in use and which will reduce the distortion in such circuits.

It is a still further object of this invention to provide a sound amplifier for motion picture theatres in which a cascade of vacuum tubes is provided in each step of amplification, each normally operating at half or less capacity so that failure of one tube will permit the remaining tube or tubes in the cascade to carry the full load and will not cause interruption of the amplification, and so that under all normal operating conditions all the equipment is being used at less than full capacity, thereby giving a high factor of 6D safety.

It is a still further object of this invention to provide an amplifier having little distortion, which is provided with a plurality of amplifying stages, each of which is provided with a plurality of vacuum tubes whereby destruction of any vacuum tube will not cause interruption in the amplifying circuit.

It is a further object of this invention to provide an alternating current and rectiiier for grid potential bias.

It is a further object of this invention to pro- Vide an alternating current supply and cooperating transformers and rectiers for furnishing all the energy for a film or disk pick-up in which there is no noticeable distortion within the audible range of frequency.

It is a still further object of this invention to provide an amplifying circuit for a synchronized talking moving picture which has a plurality of amplifying tubes in each stage of amplification, each tube normally acting at approximately onehalf or less its rated capacity so that if one tube is destroyed, the other tube or tubes will take up the full load, Without distortion or overload.

It is a still further object of this invention to provide a more efficient amplier for synchronized talking moving pictures which will be less expensive and will be more effective in use.

Further objects and advantages of this invention will appear from the following description thereof, taken in conjunction with the attached drawing, which is a diagrammatic view of one adaptation of our invention showing a preferred form of embodiment thereof.

The input in this particular illustrative embodiment is derived from a plurality of disk reproducing pick-ups 10 and 10a or a plurality of photo-electric cells l2 and 12a for the lm pickups which are connected in circuit through the fader 14 having circular segments 16 and 16a and the circuit controlling arm 18, which is in turn mechanically connected with the resistor armv 19 cooperating with the resistances 20 in the fader. The circuit controlling arm is grounded so that if the arm is on the segment 16 as shown, the pickup devices 10 and 12 are grounded. The pick-up devices 10a and 12a are, however, in circuit through the ungrounded segment 16a and through the appropriate resistances 20 in the fader 14. As the circuit controlling arm 18 reaches the lowermost position, it will contact with both segments 16 and 16a, thus grounding both sets of pick-up devices to prevent any overlap in the sound reproduction. As the arm 18 is then moved further counter-clockwise, it` Will 'groundsegment 16a and connect pick-up devices 10 or 12 and as the arm is further moved, the resistances in the fader will be further cut out by the mechanically linked resistor arm 19 until the proper Volume is reached. It is to be understood that the disk and film pick-ups are duplicated for continuous operation and that reproduction will be from only one pick-upl at a time and that the obvious switches are not shown.

This construction is very satisfactory for the purpose of transferring the pick-up from one set of reproducing or recording devices to another and simultaneously controlling the volume so that a fading out of one pick-up, and a fading in of another can be made, thereby eliminating abrupt noticeable changes which are highly objectionable. The double or continuous pick-up control is thus reduced to its simplest form, insuring a continuity of the program.

I'he input from the fader 14 then passes through the wires 21 and 22 to the primary of the audio transformer 24, and the tap 22a may be varied, as desired, for proper impedance matching. The conductor 22 is also provided with a condenser and resistance 28 which effectively balances and neutralizes the effects of the resistances 20 on the fader 14 at low frequencies to give amore exact reproduction of the incoming impulses and prevent over-accentuation of the base notes. The secondary of the transformer 24 is connected to the first stage of amplification which includes the four-element, heater-type, vacuum tubes 26 and 26a in parallel, the plates and grids having common connections. The filaments of these tubes are heated from an alternating current supply which may be reduced by a transformer not shown to approximately two and one-half (2l/2) volts. A center tap potentiometer 28 is used to neutralize the A. C. fluctuation applied to the grid. The output of this stage of amplification passes into the primary of the second transformer 24a.

- The second stage of amplification is similar to the first having the plurality of four-element, heater-type vacuum tubes 30 and 30a similarly connected in parallel with common plate circuits and common grid circuits. The filaments of these tubes are also similarly connected to the transformer and potentiometer 28 although obviously a separate connection could be used, having a separate source of alternating current.

Choke coils 32 and 32a and condensers 33 and 33a are also duplicated, one being used at the output end of each stage of amplification. It is also obvious that additional stages of amplification would be similar to the first two and could be used if necessary.

With the vacuum tubes duplicated, each tube will take substantially half the load and will therefore be used at substantially only half its rated capacity. If one of the amplifier tubes should burn out, therefore, no effect will be apparent to the audience as the output will be constant, the remaining tube assuming the full load until the defective one is replaced. In this manner, the operator does not have to be relied upon to throw a switch or connect a reserve amplier as the action is entirely automatic and, therefore there will be no break in the sound reproduction due to the changing over and warming up of the reserve unit.

It is usually necessary to use power speakers and the power amplifier stage consists in the use of three or more suitable vacuum tubes which may be the ordinary three-element tubes which are also connected in parallel, having all their plate and grid circuits respectively in common. This connection of the tubes, although operating them to triple the power amplification of the input, still allows sufficient reserve capacity, so

that if one tube should become inoperative, sufficient capacity exists in the remaining tubes to carry the full load without overloading. The added reserve obviously materially increases the reliability of the circuit and prevents failure during the program, although it may be necessary to vary the fader control slightly.

In order to produce a sufficient negative potential on the grid and to bring about the so-called bias of the grid, we resort to a separate rectified alternating current supply which has been properly transformed in an iron core transformer 36 connected to an alternating current supply usually of one hundred fifteen (115) volts. The secondary of this transformer is in circuit with a rectifier 38, choke 39 and resistance 40 which may be of any suitable value. The rectifier is shown as a crystal rectifier but obviously may be a vacuum tube rectifier or chemical rectifier or any other form.

The negative side of the bias circuit further is connected through a resistance 42 to the grids of the three power amplifier tubes 44, 44a and 44h. The positive side of this bias circuit is connected to the ground through the conductor 21.

The filaments in the power amplifier tubes may be heated in the same manner as the filaments in the first tubes from a suitable alternating current supply reduced by a transformer not shown and then through the second center tap potentiometer 46 so connected that the phase of the alternating current is equalized in each side of the coil and so that the alternating ripple of the variable potential does not affect the incoming current, or any other suitable source of current could be used.

1t will be noted that one side of the choke coil B2b is connected through the milli-ammeter 48 to the conduit 50. As is well known, the fluctuations of the milli-am'meter show the operation of the vacuum tubes and if they are being overloaded, the operator noting the effect on the milliammeter, may vary their current supply, or vary the load by means of the fader.

After the voltage has been amplified in the first amplification stages, and the corresponding energy is impressed on the primary of the transformer 24h, and then passed through the power amplifier which amplifies the current, the secondary of final transformer 24o may be tapped for the various loud speaker connections which may be arranged as desired, It is also within the scope of our invention to eliminate the final output transformer and connect the loud speakers, not shown, directly to the power amplifier output or to eliminate the transformer and choke in order to reduce the distortion by that equipment.

The operation of the device is as follows:

The input from either pick-up 10 or 12 or 10a or 12a is passed through the fader into the primary of the first transformer 24. The fader permits the operator to switch from one set of operating conditions to another without making the change appear abrupt and to control the sound volume.

The first stage of amplification which comprises two vacuuin tubes in parallel having heater elements distinct from the cathodes in order not to amplify the alternating current ripple which would be noticeable if only three-element tubes were used, includes the tubes in parallel so that injury to one will not force a shut-down of the circuit or require the switching to a duplicate, and expensive emergency amplifier.

The second stage of amplification is similar to iss the rst stage and likewise has a plurality of duplicate tubes of the four-element, separate heater-type. It is of course obvious that additional stages of amplification may be used, smilarly comprising a plurality of vacuum tubes having grids and plates in parallel, if it is necessary for greater voltage amplification. Also that an additional number of tubes may be used in each stage of ampliiication.

After the preliminary stages of amplification, the power amplifier which has the same feature of duplicate vacuum tubes which are working at substantially less than their usual capacity proportionate to the number used and also have more than suflicient reserve if one tube becomes inoperative, is connected to the appropriate transformer 24o. In these power tubes, it will be noted that the grid bias is produced by a rectiiied A. C. supply and the necessary resistance. This rectified A. C. supply may be very effectively used and the size of the resistance is found to be immaterial. It is to be understood that a vacuum tube or chemical rectifier might be used equally well in place of the crystal rectifier shown.

The alternating current supply is also very effectively used to heat the filaments by connecting through a suitable tapped transformer and a center tap potentiometer may be used as shown.

It will be noted in this circuit that once the operation is started, all the tubes are always hot and it is unnecessary to interrupt the program in the event of failure of any one of the tubes. A further advantage of the described hook-up, is the decreased cost of the single amplifier unit described as compared with the cost of duplicate emergency amplier equipment now in use together with the increased reliability and better reproduction of a plurality of vacuum tubes functioning at a portion only of their actual capacity and so arranged as to permit the assumption of the full load by the remaining tube or tubes in that stage in the event of the failure of one of the several tubes, and the resultant freedom from distortion when the tubes are operated at partial capacity.

The alternating current supply is used for heating the ilaments and biasing the grids using the necessary transformers and center tap potentiometer and necessary rectifier respectively so that an undiminished single source of alternating current supply is available. Furthermore, as the supply is available to operate all the tubes and as all the tubes are kept hot, there is no variation noticeable to the audience if a tube should cease to operate. It is understood that although diierent voltages are required for different circuits, it is possible to use a single transformer, tapping it where necessary and Where direct current is required, this may also be supplied by an appropriate rectifier.

The potentiometer together with the grid bias which may be used, in all the amplier tubes if desired, particularly permits increased operating efciency as the tubes may be operated at a much lower capacity with a greater reserve, which, .with the decreased cost of equipment, makes the duplication of circuits entirely unnecessary. At all times the entire equipment is in use and no part is out of circuit which uses the equipment to its best advantage and Without fully loading the tubes normally and without overloading any tube in the event of failure of any tube.

While we have shown a preferred form of embodiment of our invention, we are aware that modications may be made therein and changes 100 resorted to and we therefore desire a broad interpretation of the invention within the scope and spirit of our disclosure and of the claim appended hereinafter.

We claim:

An amplifier of the class described for amplifying the energy picked up from a pick-up device for use in talking moving picture equipment, which comprises a series of stages of amplification, a power stage of amplication, a plurality of 110 vacuum tubes in parallel circuit in each or said stages, said power stage having three or more tubes therein, means to energize the filaments, all of the vacuum tubes in the operative condition of said amplier, and means to normally load 115 said vacuum tubes an amount proportional to their operative capacity, said amount being inverse to the number of tubes in any particular stage, so that the failure of any particular tube in any or all stages of amplification will not over- 120 load the remaining tubes and will not cause distortion.

CLARENCE A. TILESTON. RUFUS H. CALDWELL. 

